Hundreds of Salt Deposits Spotted on Mars

Anne Minard
for National Geographic News
March 20, 2008
Evidence of a dense brine that once oozed on Mars could bring new vigor to the search for salt-loving life-forms on the red planet.

Mikki Osterloo of the University of Hawaii and colleagues have discovered hundreds of small depressions that appear to be filled with the kinds of salt deposits that form on Earth when water evaporates.

The find gives more credence to theories of a wet Martian past—and it confirms the presence of chloride minerals on Mars, which some researchers have suspected for years but only now have the tools to find.

"I think the most exciting aspect of the results of this initial study is that we are continuing to discover new materials on the Martian surface," Osterloo said.

And that opens doors in the search for life uniquely adapted to Mars.

Osterloo's team presents its findings in this week's issue of the journal Science.

Dotting the Surface

The research team found the chloride deposits by studying images taken in 2001 by the Mars Odyssey Thermal Emission Imaging System (THEMIS), as well as supporting data from the Mars Global Surveyor and the Mars Reconnaissance Orbiter.

These spacecraft are sending back the highest-resolution data yet from Mars, and scientists have been improving their techniques for teasing out subtle differences in surface composition from these images.

(See the first hi-res color images of Mars taken by the Mars Reconnaissance Orbiter.)

So far the team has found about 200 deposits, all of which are smaller than 10 square miles (26 square kilometers). The features appear to be widespread across the Martian globe.

The minerals show up most often in regions that formed about 3.8 billion years ago, though some apparently formed a few hundred thousand years later.

On Earth chlorides are formed as the surfaces of large bodies of water such as lakes evaporate, leaving behind solid materials that had been dissolved in the water.

The minerals can also take shape as underground brines associated with volcanic activity evaporate.

Researchers think that saline minerals could have formed on Mars as acidic fluids evaporated during the weathering of volcanic rock known as basalt.

But there's more to be done to confirm the salt deposits and what they might mean for liquid water on the red planet, Osterloo said.

"In order to support the theory of an early wet Mars, I think we need to continue to send ground truth missions as well as orbital instruments," she said.

"About Time"

The study results are already getting a warm reception among Mars experts.

"It's about time they teased salt deposits out of the remote-sensing data!" Paul Knauth, a geologist at Arizona State University who was not affiliated with the new study, wrote in an email.

"Gary Brass showed over 25 years ago that concentrated brines could be stable there," he added. Brass, then a researcher with the University of Miami, published an oft-cited paper about brines on Mars in the April 1980 issue of the journal Icarus.

"Don Burt and I published papers several years ago showing why such brines should be there, possibly even seeping out today," Knauth continued, referring to an Arizona State colleague.

But salt didn't show up in the initial remote-sensing data, so the ideas didn't get much purchase, he said—until now.

"It is an extremely significant result, but is not surprising to those very few of us who have been howling in the wilderness about salt and brine on Mars."

Knauth hopes researchers will keep looking for more chlorides, particularly in the Hellas Basin, where he thinks dense brines could still be actively oozing.

A March 3 image of the basin taken by the Mars Reconnaissance Orbiter's HiRISE camera shows the distinctive swirling patterns of salt-marsh deposits.

"It's possible that some of these deposits are mushy right now," Knauth said, which could have "considerable astrobiological importance."

Study co-author Phil Christensen, also at Arizona State, agrees.

"The other good thing about salt is that it is an excellent preservative of organic material," he added.

"So if there was life, or even organic precursors to life, the organic molecules could be preserved in the salt deposits."

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